The marine airgun and seismic survey methods associated with the marine airgun are described in U.S. Pat. Nos. 3,249,177; 3,379,273; and 3,653,460 of the present inventor. The reader may refer to these patents for background information. Multiple airguns have been used for numerous years by geophysical exploration companies and petroleum companies. It is desired that the multiple airguns all be towed at the same predetermined level below the water surface. This has been accomplished in the past by attaching each individual airgun by a cable to an individual buoy or float, so that when at rest in the water, each airgun hangs suspended on a vertical axis beneath its respective support float. The floats, support cables, and airguns were connected together in an array (including air lines and electrical control lines) which was towed behind the survey vessel, for example, as shown in U.S. Pat. No. 3,893,539 of Lewis M. Mott-Smith. The interconnected floats, airguns, cables, airlines, and electrical lines tend to become snarled and tangled, are awkward to handle in use and are heavy and very difficult to hoist up on board the ship when not in use and again are difficult to launch into the water in unsnarled condition in readiness for use. Another U.S. Pat. which shows each airgun connected by a cable to its individual float is No. 3,437,170 of F. A. Brock and Roy C. Johnston. In this latter disclosure, the first two airguns in the array are suspended from a single float. Because of the awkwardness and difficulties of handling such complex arrays of floats, cables, airguns, air lines and electrical lines at sea under the wave conditions often encountered, there were inevitable snarl-ups and breakages, leading to expensive "down time" for makeshift repairs at sea.
Also, the complex network of multiple floats, support cables, airguns, air supply lines and electrical cables presented a very great drag and turbulence when towed through the water. This drag and turbulence imposed a great stress on the towing gear, leading to breakages and restricting the permissible forward speed of the survey vessel. These difficulties were burdensome to the operating personnel at sea, caused breakdowns, and imposed a practical limit on the numbers of airguns to be simultaneously towed.
The level of the airguns below the water surface was fixed by the length of the cables used to suspend each airgun. If the user desired to change the level of the towed airguns, it was necessary to haul all of the airguns and floats on board the vessel to handle and change the length of each support cable and then to return the whole rig of floats, cables and airguns back into the water. A slight amount of depth control could be obtained in the prior art by changing the speed of the survey vessel, thereby causing the support cables extending from the floats down to the airguns to hang at an angle as a function of the drag of the water which caused the airguns to swing back in the water and to hang at an angle below and behind their respective floats. However, this was only a limited degree of depth control since the speed of the survey vessel was not independent of other parameters and the support cable length was fixed.
For many years, many of these same geophysical exploration companies and petroleum companies have been using a long flexible hydrophone cable often called a "streamer" or a "Paslay streamer," as shown in U.S. Pat. No. 2,465,696. Such streamers contain numerous hydrophones and may extend for a distance of more than one kilometer behind the towing vessel and, when not being used, can be reeled up around a large drum on the vessel. These hydrophone streamers have been made neutrally buoyant, for example, U.S. Pat. No. 2,791,757, issued May 7, 1957, to Francis G. Blake, Glenn A. Schurman and Paul M. Aagaard discloses a neutrally buoyant hydrophone streamer having the same cross section throughout its entire length for causing a minimum of water turbulence as it is towed behind a boat. The buoyancy of this Blake, et at. hydrophone streamer can be changed by means of a small oil-filled resilient tube. The amount of oil in this internal tube can be varied from an amount which stretches the tube to an amount which leaves the tube partially collapsed for varying the overall weight of the hydrophone streamer for making it neutrally buoyant in spite of changes in water density caused by changes in temperature and salinity. Hydrophone streamers are also shown in U.S. Pat. No. 3,378,815 of the present inventor, and U.S. Pat. No. 3,471,827 of Paul Chelminski and the present inventor.
Thus, airguns have been in use since the early 1960's while hydrophone streamers have been used at least since the early 1950's by these same companies for marine seismic surveying. Insofar as I am aware, no one in any of these organizations has suggested that multiple airguns may be handled and towed in a flexible, inflatable, buoyancy-controllable streamer.